Techniques for force sensing in additive fabrication and related systems and methods

ABSTRACT

Techniques for force sensing in additive fabrication are provided. According to some aspects, an additive fabrication device may include a force sensor configured to measure a force applied to a build platform during fabrication. A length of time taken for a layer of material to separate from a surface other than the build platform to which it is adhered may be determined based on measurements from the force sensor. Subsequent additive fabrication operations, such as subsequent motion of the build platform, may be adapted based on the determined length of time.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Patent Application No. 62/679,167, filed Jun. 1, 2018and U.S. Provisional Patent Application No. 62/817,293, filed Mar. 12,2019, which are hereby incorporated by reference in their entireties.

BACKGROUND

Additive fabrication, e.g., 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography, selective or fuseddeposition modeling, direct composite manufacturing, laminated objectmanufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build parts by forming successive layers, which are typicallycross-sections of the desired object. Typically each layer is formedsuch that it adheres to either a previously formed layer or a substrateupon which the object is built.

In one approach to additive fabrication, known as stereolithography,solid objects are created by successively forming thin layers of acurable polymer resin, typically first onto a substrate and then one ontop of another. Exposure to actinic radiation cures a thin layer ofliquid resin, which causes it to harden and adhere to previously curedlayers or the bottom surface of the build platform.

SUMMARY

According to some aspects, an additive fabrication device configured toform layers of solid material on a build platform is provided, theadditive fabrication device comprising a container, a build platform, atleast one force sensor configured to measure a force applied to thebuild platform, and at least one processor configured to form a layer ofmaterial in contact with the container, and in contact with the buildplatform and/or a previously formed layer of material, measure, usingthe at least one force sensor, a length of time taken to separate thelayer of material from the container, and control motion of the buildplatform based at least in part on the measured length of time taken toseparate the layer of material from the container.

According to some aspects, a method is provided of operating an additivefabrication device configured to form layers of solid material on abuild platform, each layer of material being formed in contact with acontainer in addition to the build platform and/or a previously formedlayer of material, the method comprising forming a layer of material incontact with the container, and in contact with the build platformand/or a previously formed layer of material, measuring, using at leastone force sensor, a length of time taken to separate the layer ofmaterial from the container, and controlling motion of the buildplatform based at least in part on the measured length of time taken toseparate the layer of material from the container.

The foregoing apparatus and method embodiments may be implemented withany suitable combination of aspects, features, and acts described aboveor in further detail below. These and other aspects, embodiments, andfeatures of the present teachings can be more fully understood from thefollowing description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIGS. 1A-1D depict an illustrative stereolithographic device and stagesof its operation, according to some embodiments;

FIGS. 2A-2B depict an illustrative exposure module that includessegmented rollers, according to some embodiments;

FIG. 3 depicts a film placed under tension along a single axis,according to some embodiments;

FIG. 4 depicts a film placed under uniform tension in multiple axes,according to some embodiments;

FIG. 5 depicts a film placed under a range of tensions from one side ofthe film, according to some embodiments;

FIG. 6 depicts an illustrative tank for a stereolithography device thatincludes a multiple films, according to some embodiments;

FIGS. 7A-7B depict two illustrative approaches to directing light withinan exposure module, according to some embodiments;

FIG. 8 depicts an illustrative adjustable tension system, according tosome embodiments;

FIG. 9 depicts a second illustrative adjustable tension system,according to some embodiments;

FIG. 10 depicts an illustrative linear motion system that includeposition sensing elements, according to some embodiments;

FIG. 11 depicts an illustrative example of curing a layer by scanningover a build region whilst activating and deactivating a light source,according to some embodiments;

FIG. 12 is a block diagram of a system suitable for practicing aspectsof the invention, according to some embodiments;

FIG. 13 depicts a measurement of force measured by a position sensingsystem over time, according to some embodiments;

FIGS. 14A-14B depict a method of sensing a position of a roller of anexposure module, according to some embodiments; and

FIG. 15 illustrates an example of a computing system environment onwhich aspects of the present application may be implemented.

DETAILED DESCRIPTION

The inventors have recognized and appreciated improved techniques forstereolithography, including improved systems and methods. As discussedabove, some additive fabrication techniques may form solid objects bysuccessively forming thin layers of material on a build platform. Instereolithography, such layers of material are formed from a liquidphotopolymer. Light is directed to a selected portion of the liquidphotopolymer, thereby curing it to a solid (or semi-solid) layer in adesired shape.

Some stereolithographic devices may form solid material in contact witha surface additional to previous layers of the part or the buildplatform, such as a container in which liquid photopolymer is held. Inthese cases, actinic radiation (radiation that initiates and/or developsthe curing process) may be introduced through an optical window in thebottom of a liquid photopolymer container. These types ofstereolithographic devices are sometimes referred to as “invertedstereolithography” or “constrained surface stereolithography” devices.

Since, in an inverted stereolithography device, liquid photopolymer iscured in contact with a surface other than the part being fabricated,the cured photopolymer must be separated from that surface beforesubsequent layers of the part can be formed. Multiple problems mayarise, however, due to the application of force during separation of thepart from the container or other surface. In some use cases, theseparation process may apply a force to and/or through the part itself.A force applied to the part may, in some use cases, cause the part toseparate from the build platform, rather than the container, which maydisrupt the fabrication process. In some use cases, a force applied tothe part may cause deformation or mechanical failure of the part itself.In some cases, forces applied to a part during separation processes canbe reduced by forming the part in contact with an upper surface of amaterial with properties that assist in physical separation of the partfrom the material. A layer of this type of material is sometimes calleda “separation layer.”

“Separation” of a part from a surface, as used herein, refers to theremoval of adhesive forces connecting the part to the surface. It maytherefore be appreciated that, as used herein, a part and a surface maybe separated via the techniques described herein, though immediatelysubsequent to the separation may still be in contact with one another(e.g., at an edge and/or corner) so long as they are no longer adheredto one another. Adhesive forces may include chemical forces (e.g., bondscoupling the two materials together) and/or mechanical forces (e.g.,fluid dynamics and/or vacuum pressure).

Conventional separation layer approaches include thin film approaches inwhich a film is suspended or otherwise extended over an area to formsome or all of a base of a container, and approaches in which a releasecoating, such as silicone, is applied to the interior of a rigidcontainer. In both cases, the aim is to reduce the adhesive forcesbetween the part and the separation layer (e.g., the film or the releasecoating) to lower the amount of force that must be applied to separatethe part from the separation layer. In thin film approaches, theflexibility of the film can be leveraged to more readily induce a peelof the film from the part as compared with rigid containers containing arelease coating. There are additional challenges, however, with the thinfilm approach that generally are not encountered with the container andrelease coating approach.

First, it is highly desirable in stereolithography to form solidmaterial in layers that are sufficiently flat to form the layers of thepart in desired shapes and so that the layers of the part stack cleanlytogether. In the case of inverse stereolithography, this requires asufficiently flat separation layer. In the case of a rigid container andrelease coating, the release coating can be arranged to be flat since itis attached to a flat, rigid container. In the thin film approach,however, the film is typically suspended over an opening and may sag orotherwise form a non-planar surface such that layers formed in contactwith the film are not formed on a sufficiently flat surface. Sincelayers of material in stereolithography are often formed withthicknesses of hundreds of microns or tens of microns, even a smalldeviation of the thin from a flat state can negatively affect thedesired flatness of fabricated layers.

Some conventional stereolithography devices have employed one or morerollers to push a film into a flat state during fabrication. Theinventors have recognized and appreciated that such rollers, however,must be produced to a very high tolerance to consistently produce flatsurfaces at the same height. This tolerance includes both the degree towhich the cross-section of a roller is circular, since any ellipticityor other deformity could produce inconsistent film heights, and thedegree to which the diameter of the roller varies across its length. Theinventors have recognized and appreciated that these tolerances can beas small as a few microns. In some cases, at least some of the rollersmay have fine or smooth surface finishes, to limit friction against thefilm and reduce the potential for wear, tear or puncturing of the film.

Second, the above solutions for producing a flat surface in a thin filmgenerally apply some form of tension to the film, whether across thewhole film to produce a flat, or close to flat, surface, and/or bydeforming the film using one or more rollers. These tensional forces canfatigue the film material over time, and wrinkles or other non-planardeformations may be produced in the film after the repeated applicationof such forces. In many cases, the defects may have altered the elasticproperties of the film enough that the application of additional (orreduced) tension cannot mitigate these defects to produce a sufficientlyflat film surface.

Third, it is desirable that a film is relatively permeable to thediffusion (or other transport) of oxygen and/or other gases through thefilm. These gases can inhibit curing of the liquid photopolymer at theliquid/film interface, leading to uncured and/or partially curedphotopolymer at the interface, which reduces the forces needed toseparate the film from the cured layer of the part. At the same time, itis desirable for the film to be relatively impermeable to thephotopolymer materials, as otherwise those material might causeundesirable changes to the film, such as degradation of the mechanicalor optical properties of the elastic material. Photopolymer materialsthat may cause any undesirable changes in the film due to interactionsbetween the two materials are referred to herein as being “incompatible”with the film. For example, certain substances, such asisobornylacrylate, have been found to cause PDMS to expand, “swell” oreven separate from other materials. This behavior may render a PDMSseparation layer in a stereolithographic printer unusable. As such,those substances may be referred to as being incompatible with PDMS.

As a result of the above challenges, the choice of materials for afilm-based stereolithography separation layer has tended to favorproperties such as mechanical strength, over other properties ofinterest, such as degree of optical transmissivity and oxygenpermissibility. For instance, films have conventionally been constructedfrom materials in the Teflon® family, and/or from otherpolytetrafluoroethylene-based formulae. While such materials provide foronly limited oxygen diffusivity and actinic transparency, the mechanicalproperties of such materials allow for sufficiently thin films to beutilized in order to partially compensate for such deficits.

The inventors have recognized and appreciated techniques for mitigatingthe above-described challenges regarding film-based stereolithographyseparation layers. In particular, the inventors have realized a filmapproach that includes multiple individual films, film tensioningtechniques, and segmented rollers. Taken individually or in any suitablecombination, these improvements mitigate at least one of theabove-described challenges, as will be described in further detailbelow.

Another problem that may arise in stereolithographic devices that use alaser light source is that the laser beam must be directed to variouspositions within the build volume, which are generally positioned atdifferent distances from the laser source. Thus, the optical path lengthfrom the laser source to the location at which liquid photopolymer is tobe cured will vary across the build volume. Yet laser beams and theirassociated optics do not always produce a well-defined spot of light ata wide range of optical path lengths, and consequently directing thelaser beam to exterior regions of the build volume may result in solidmaterial being formed in those exterior regions in a less precise manner(e.g., due to the spot of light being less distinct). In manystereolithographic devices, this limitation of a laser source places apractical upper limit on the size of the build volume. Some conventionalstereolithographic devices may employ a digital light processing (DLP)source as the light source, which can produce light that has the sameoptical path length to all points in the build volume and can expose alarger portion (e.g., all) of a build area to actinic radiationsimultaneously. This can, in at least some cases, reduce overall buildtime. DLP light sources, however, contain a fixed array of light sourcessuch that their light is directed only to fixed locations within thebuild volume, such that there may be locations in the build volume towhich light cannot be directly applied or cannot be applied with desiredaccuracy. Furthermore, as the build volume increases, the accuracy ofthe DLP light source decreases as the light must travel a longerdistance and may diverge over the longer distance.

The inventors have recognized and appreciated that a light source thatcan be moved across the build area would mitigate the above-describedissues by allowing light to be directed to any desired location withinthe build volume by moving the light source. This also allows thedistance from the light source to the build volume (the optical pathlength) to be substantially the same for each position across the buildarea by moving the light source whilst maintaining a fixed distance fromthe light source to the build volume. This configuration may allow forfabrication of larger parts in a stereolithographic device byeliminating the practical upper limit on the area of the build volumethat can be imposed by use of a laser light source, as discussed above.In some embodiments, a moveable light source may be arranged with one ormore rollers in a common unit, or “moveable stage.”

In some embodiments, a moveable stage may include a light source thatmay be directed along a single axis, such that a combination of movementof the moveable stage and directing the light along the axis may allowdirection of light to any desired location within a build region. Insome embodiments, the moveable stage may move at a constant speed acrossthe build region whilst the light is directed back and forth along thesingle axis. In this manner, a layer may be cured in a series of scanlines running from one side of the build region to the other.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, improved techniques forstereolithography. It should be appreciated that various aspectsdescribed herein may be implemented in any of numerous ways. Examples ofspecific implementations are provided herein for illustrative purposesonly. In addition, the various aspects described in the embodimentsbelow may be used alone or in any combination, and are not limited tothe combinations explicitly described herein.

An illustrative stereolithographic device and stages of its operationare depicted in FIGS. 1A-1D, according to some embodiments. As shown inthe example of FIGS. 1A-1D, stereolithographic device 100 includes abuild platform 105 that is configured to adjust its position towards andaway from tank 104 along an axis 106, referred to herein as the Z axis.The build platform 105 may support a part 101 being formed by thestereolithographic process.

In the example of FIGS. 1A-1D, the tank 104 may contain a volume ofphotopolymer resin 102 and comprise a bottom surface formed by a thin,flexible and/or elastic film 103, substantially transparent to actinicradiation 115. The film 103 may be held under tension by a tensioningdevice 107. An exposure module 109 may be moved along axis 108, referredto herein as the X axis, such that roller elements 111 are in contactwith the lower surface of the film 103. The exposure module 109comprises an exposure source 110 of actinic radiation 115 whichselectively emits actinic radiation along its length (i.e., the axisrunning orthogonally to both axis 106 and 108, referred to herein as theY axis). The exposure module 109 further comprises roller elements 111which are mounted to the top side of the exposure module 109 opposingthe bottom of the film 103.

In some embodiments, the film 103 may comprise any highly flexibleand/or non-reactive material, such as Teflon® (or other fluoropolymer orpolytetrafluoroethylene-based material, such as fluorinated ethylenepropylene). The sides of the tank 104 may be comprised of a more rigidmaterial, such as an acrylic plastic, or may alternatively may be formedof a flexible or compliant material.

According to some embodiments, the stereolithographic device 100 may beoperated to fabricate an object, or part, 101 by selectively solidifyinglayers of photopolymer resin 102 onto build platform 105 by exposing thephotopolymer resin 102 to a source 110 of actinic radiation 115. Inparticular, as shown in FIG. 1A, the build platform 105 may be movedalong axis 106 to place the bottom of the build platform 105 or mostrecently formed layer of the part 101 in close proximity to the bottomplane of the tank 104 and the film 103. As the bottom film 103 typicallyhas a certain degree of flexibility and/or elasticity, the weight of thephotopolymer resin 102 and/or downwards pressure from the motion of thebuild platform 106 and part 101 may cause the film 103 to form a “sag”112, or other form of depression.

In the example of FIG. 1B, exposure module 109 has been moved along thebottom plane of the tank 104 through axis 108. During this motion,roller elements 111 may press upwards against film 103 in order toflatten any deflection in the film and ensure that the film forms asubstantially flat plane between the roller elements in contact with thefilm. Also during the motion, an exposure source 110 may be activated inorder to cause actinic radiation 115 to be selectively emitted atvarious points along the Y axis. Actinic radiation 115 emitted by theexposure source 110 may be transmitted through the film 103 andirradiate a layer of photopolymer resin 102 located between the film andthe lower surface of the part 101. When exposed to the actinic radiation115, the exposed portion of the photopolymer resin 102 may undergovarious reactions, such as polymerization, causing the flowable resin102 to solidify or otherwise adhere to the previously formed layer ofthe part 101, forming a new layer 114 of the part 101. As shown in theexample of FIG. 1C, the exposure module 109 may continue to move alongthe X axis while selectively exposing regions along the Y axis using theexposure source 110. Accordingly, any desired region within the X-Yplane of the bottom of the tank 104 may be selectively exposed toactinic radiation, causing polymerization of a new layer 114 of the part101 in the desired shape.

Following exposure, the newly formed layer 114 may be in contact withboth a previously formed layer and the film 103. While adhesion isdesirable between the newly formed layer 114 and the prior layers of thepart 101, unwanted adhesion may also be formed between the newly formedlayer 114 and the film 103. As discussed above, an additional step istypically required to break such adhesive forces before the formation ofa new layer, in a process referred to herein as “separation.”

As shown in the example of FIG. 1D, one way of performing separation inillustrative stereolithographic device 100 is to lift the part 101, andthus newly formed layer 114, along axis 106, away from the film 103.Adhesive forces between the newly formed layer 114 and the film 103 maycause the film to deflect upwards 115 as the build platform 105 is movedaway. Using a flexible, thin film as at least part of the floor of thecontainer may allow a peeling edge to propagate inward from most or allof the outer edge of the contact area between the part 101 and the film103. In particular, at a critical level of deflection, at least oneportion of the film 103 may begin to separate, or peel, away from thenewly formed layer 114, thus forming a peeling edge 116 which propagatesacross the interfacing surface of the film 103 and newly former layer114. Separation of this manner may apply considerably less force to thepart 101 compared with separation of a part from a rigid containerhaving a release coating, as discussed above.

Following separation pictured in FIG. 1D, a new layer of the part 101may be formed by returning to the configuration shown in FIG. 1A. Insome embodiments, this may comprise returning the exposure module 109 toits original position (as in FIG. 1A) without forming additional solidmaterial. In other embodiments, however, the direction of the exposuremodule 109 along axis 108 may be reversed, such that the formationprocess depicted in FIGS. 1A-1D occurs with the exposure module 109moving in the opposite direction.

According to some embodiments, contact between the roller elements 111and the bottom of the film 103 may help to form a flat surface of thefilm 103 against which the formation process of a new layer 114 mayoccur. In some embodiments, the span between the roller elements 111(distance between them along the X axis) may be 20-80 mm, but ispreferably 40 mm. In some embodiments, the roller elements 111 mayextend above the exposure module 109 sufficiently to ensure that thefilm 103 does not come into contact with the exposure module 109, otherthan via the roller elements 111. In some embodiments, 1-3 mm may besufficient to achieve this result, but the extension may vary, in partdue to the span between the roller elements 111.

The inventors have recognized and appreciated that the span between theroller elements 111 may limit or otherwise impact the maximum speed atwhich a new layer 114 may be formed. In particular, following exposure,photopolymer resin may continue to undergo initial chemical reactionsfor a period of time. During this period, the photopolymer resin may nothave reached a sufficient degree of solidification or other physicalproperties to resist subsequent forces applied by the fabricationprocess. During the transit of the roller elements 111, comparativelylittle downwards force may be exerted against the newly formed layer 114material. Following the passage of the trailing roller element (e.g.,the leftmost roller element in the example of FIGS. 1A-1D), however,such forces may increase because at this point the film may begin tosag. Accordingly, it may be advantageous to ensure that the trailingroller element 111 does not reach a section of newly formed layer 114until a sufficient delay period (referred to herein as a “post-curedelay period”) has elapsed. It may thereby be seen that the distancebetween the roller elements 111 may limit the potential rate at whichthe roller elements 111 may advance and that a larger span may allow formore rapid advance because the same post-cure delay can be produced yetat a faster speed.

A larger span between the roller elements 111, however, riskscompromising the basic function of the roller elements to support thefilm 103 and to ensure a flat surface for the formation of a new layer114. These competing interests may be resolved, however, by the additionof additional roller elements 111. As an example, in some embodiments athird roller element may be added behind the otherwise trailing rollerelement such that exposure may continue to occur between the first tworoller elements 111, while a third roller element trails in order toensure that the newly exposed material has sufficient support for adesired post-cure delay period. Such a third roller element may form aspan of greater, lesser, or equal length to the first span, dependingupon the amount of additional delay desired. Alternatively, the spacingbetween the roller elements 111 may be adjustable, whether manually orthrough active means, depending upon the extent to which additionalpost-cure delay is desirable and the desired speed of traversal of theroller elements 111. In one such example, one or more roller elements111 may be configured to be moved separately from the exposure module109, such that a leading or trailing roller element may be placed intocontact with the film 103 with timings independent from the motion ofthe exposure module 109.

In some cases, it may be advantageous for certain roller elements 111 tobe mounted or otherwise positioned with different Z axis offsets fromthe bottom of the film 103. As one example, in some embodiments one ormore trailing rollers may be mounted with an offset below the bottom ofthe film 103 to avoid excess compression of newly cured material formedon the opposing side of the bottom of the film 103, whereas one or moreleading rollers may be mounted at a higher Z axis position, such as atthe bottom of the film 103. This higher position may be attained byspecifically manufacturing an asymmetrical mount for the two rollers, ormay be achieved with the use of additional adjustment features or shims.In such embodiments, the configuration of the roller elements, and thusexposure module 109, may not be symmetric with respect to the directionof motion along axis 108. In such a case, the exposure module 109 may berepositioned between layer formation in order to return to a specificstarting position, rather than continued formation of a second layerwith a reciprocal motion.

In contrast to the pictured roller elements of FIGS. 1A-1D, someconventional stereolithography devices may include elements other thanrollers may be used in order to support the film, such as a static lip,circular bearings, or other protrusions. It is, however, typicallyadvantageous to reduce the amount of friction or other lateral forcesexerted by the motion of the exposure module against the film becausesuch forces may scratch or tear the film. In some cases, conventionalstereolithography devices apply low surface energy “non-stick” materialsor other lubricious materials to the protrusions to reduce friction.

Rollers possess a number of potential advantages compared with staticprotrusions, including both a minimal profile for the contact areabetween the element and the film and the ability to “roll,” rather thanslide, in response to any frictional forces exerted by the film. Asdiscussed above, however, rollers may be disfavored overall because ofthe demand for small tolerances so that flat surfaces are consistentlyproduced at the same height. Such exacting tolerances may be difficultand expensive to meet in practice. The inventors have, however,recognized and appreciated a roller element design that can produce asufficiently flat film surface without it being necessary for eachcomponent of the roller elements to individually be produced at suchsmall tolerances.

FIGS. 2A-2B depict an illustrative exposure module that includessegmented rollers, according to some embodiments. In the example ofexposure module 209 shown in FIG. 2A, each of the two pictured rollerelements 211 comprise four roller segments 212. Exposure module 209 maybe included as exposure module 109 in the illustrativestereolithographic device of FIGS. 1A-1D such that the roller segments212 extend along the Y axis of the device 100.

In the example of FIGS. 2A-2B, roller segments 212 may each be formedfrom any suitable material or materials, but may preferably comprisecomparatively incompressible and wear resistant materials, such asaluminum, stainless steel (e.g., 303 grade stainless steel), chromesteel, and/or other grades of steel commonly used for bearings. In someembodiments, roller segments 212 may be treated with one or more coatingmaterials, such as one or more ceramics, titanium nitride, chrome, orcombinations thereof. In some implementations, roller segments 212 mayinclude bearing steel rods of approximately 3-10 mm, with each rodhaving a length of between 20 mm and 60 mm. In some implementations, theexposure module 209 may include four roller segments within each rollerelement, with the segments having individual lengths of 45 mm and adiameter of 6.35 mm. Although shown to be of equal lengths in theexample of FIGS. 2A-2B, the lengths of the roller segments may ingeneral differ from one another. It will be appreciated that the use offour roller segments in the illustrative roller elements is provided asone example, and any number of roller segments 212 may in general bearranged within each roller element 211.

In the example of FIG. 2A, spacers 217 are provided to maintain apredefined separation distance between roller segments 212, which mayact to prevent wear and other contact interactions between rollersegments 212. In some embodiments, such spacers may be flexiblecouplings, such as silicone adhesive connecting roller segments 212. Insome embodiments, spacers 217 may include an independently movableelement, such as a ball bearing.

As shown in the example of FIG. 2B, a roller segment 212 may be disposedwithin a retaining feature 218. According to some embodiments, theroller segment may be attached to the retaining feature or to some otherpart of the exposure module 209, or may be held in the retaining featurewithout attachment. Retaining features 218 may act to limit the range ofmotion of roller segments 212. In particular, as shown in the example ofFIG. 2B, the retaining feature 218 may include overhangs or “fingers”220, which may limit the lateral and/or upward range of motion of theroller segment 212. The retaining feature 218 may further include asupporting base portion 219 that limits the downwards range of motion ofthe roller segment. In some embodiments it may be advantageous to addadjustment features such as shims to ensure the rollers are maintainedat a desirable height or position. Such adjustment features may furtherreduce impacts of manufacturing tolerances.

In some embodiments, retaining features 218 may comprise a low-frictionand/or wear-resistant material, such as nylon, polyacetal,polytetrafluoroethylene (PTFE), ultra-high-molecular-weight polyethylene(UHMWPE), and/or PEEK, allowing the roller segment 212 to move againstthe fingers 220 and supporting base 219. It should be noted, however,that it is not necessary for the roller segments 212 to actually rollcontinually or at all during operation, though such rolling may reducethe lateral forces exerted against a film. In general, the inventorshave found that a small amount of clearance between the retainingfeatures 218 and roller segments 212 to be advantageous. In particular,a clearance between the retaining features 218 and roller segments 212may be between 10 μm and 50 μm, or between 20 μm and 40 μm, or less than50 μm.

In some embodiments, the roller segments 212 may have less than 200 μmof clearance to move in the Z axis against the fingers 220 of theretaining features 218 and less than 50 μm of clearance to move in the Xaxis against the sides of the retaining features 218. In someembodiments, roller segments 212 may be constrained from motion awayfrom the supporting base 219 by the tension of a film in contact withthe roller segment 212.

In some embodiments, fingers 220 may extend over the roller segment 212a sufficient amount to restrict the motion of the roller segment. Inaddition, or alternatively, ends of roller segments 212 may be shaped invarious ways to minimize unwanted interactions between segments adjacentto one another along the Y axis. In some embodiments, a cylindricalspacing region may be formed at the ends of each roller segment, thespacing regions having a diameter smaller than the diameter of thenon-spacing regions of the roller. In some embodiments, a narrowedpositive feature formed by such a narrowed cylindrical segment may bepaired with a negative feature in the abutting roller formed by acylindrical recess, thus partially interlocking the roller segments 212.The sides of the roller segments 212 may further incorporate chamfers,bevels, and/or other features so as to avoid sharp edges that mayincrease wear against the film 103. According to some embodiments, anyfilm contacting edges of the rollers may be polished so as to avoid wearor deformation of the film.

The inventors have recognized and appreciated that roller segments 212allow for the roller segments to have significantly larger dimensionaltolerances, such as straightness or average diameter, compared with thetolerances usually necessary for a roller element to produce a desiredfilm flatness. As discussed above, a roller elements generally demandsmall tolerances so that flat surfaces are consistently produced at thesame height. A roller element comprising roller segments may, however,produce a consistently flat film surface even though a consistently flatfilm surface would not result if the same cylindrical material were usedas a single piece roller element.

For instance, small deviations in straightness, such as a bend, in asingle long roller may result in a significant displacement of thesurface of the roller from the midline at the midpoint of the roller.The same degree of deviation in straightness, however, in a shorterlength of roller, may result in a much smaller total displacement in thesurface of the roller from the midline of at the midpoint of the shorterroller. Accordingly, the use of multiple, and thus shorter, rollersegments 212, allows for smaller total displacements, even with the sametolerances in straightness. A much wider range of tolerances maytherefore be acceptable in the roller segments 212. In other words, thedimensional tolerance of the retaining features, particularly withregards to the supporting base 219, may be the primary influence on theprecision and accuracy of the motion of the roller segments, rather thanthe dimension tolerances of the roller segments themselves. Theprovision of a uniformly flat and level supporting base 219 (withrespect to the XY build plane), however, may be considerably easier andless expensive.

As an alternative to the depicted segmented cylindrical roller segments212, in some embodiments, one or more different segment structures maybe combined to form a roller segment. For instance, circular ballbearings and/or flexible rods may be arranged in place of theillustrative cylindrical roller segments. Conceptually, a sufficientlyflexible rod may decouple the deflection and/or deviation at a givenportion of the rod from more distant points on the rod. In someembodiments, an otherwise inflexible rod may be modified by the additionof circular cuts spaced along the length of the rod. As one example, arelatively inflexible rod having a diameter of 6.35 mm and length of 200mm may be modified by making radial cuts, or trenches, of approximately2 mm into the rod spaced 40 mm apart along the length of the rod. Theremaining core of the rod, having a diameter of 2.35 mm, may becomparatively more flexible than the full width rod and allows for aform of segmentation, whereby unmodified regions of the rod locatedbetween trenches are capable of a decoupling deflection at the regionsthinned by trenches.

In some embodiments, roller segments 212 may be supported and/orinterconnected along a common axis. As one example, roller segments 212may include a cylindrical hole running lengthwise through the segments212 and a mounting device, such as a thin rod or flexible wire, may runthrough a group of segments 212 through such a cylindrical hole.Alternatively, or additionally, roller segments 212 may include a seriesof protrusions and depressions on abutting ends, such that a protrudingportion of a first roller segment 212 may extend partially into adepressed portion of an adjacent roller segment 212.

In the example of FIG. 2A, exposure module 209 further comprises anexposure source 210 located between roller elements 212. The exposuresource 210 may be configured to selectively expose a photopolymer toactinic radiation along the long axis of the exposure source 210,representing the “Y” or “fast” axis of the fabrication device. Duringoperation, the exposure source 210 may be progressively moved along the“X” or “slow” axis by the motion of the exposure module 209, asdescribed above. The combination of selective exposure along the “Y”axis and progressive motion along the “X” axis thus allows for selectiveexposure of arbitrary points within the plane formed by the “X” and “Y”axis. In some embodiments, the exposure source 210 may be capable ofexposing multiple locations, or the full width of, the “Y” axissimultaneously. This approach may be referred to as “linear” exposure,as the exposure generated by such an exposure source 210 may take theform of lines or line segments.

An example of one such exposure source may be a linear array of lightemitting elements, such as an LED “bar”, or similar structure. Asanother example, a beam of light may be projected onto a rotatingpolygonal mirror from a light source such as a laser. As the polygonalmirror rotates (e.g., at a constant angular (rotational) speed), thebeam may be deflected with a known trajectory, and is therefore directedto, and is incident on, known points. Accordingly, the light source maybe activated or deactivated based upon the known rotational position ofthe polygonal mirror, depending on whether the known path would directthe light source to a known point that is intended to be exposed.

In some embodiments, exposure source 210 may produce a coherent beam oflight. For example, the exposure source may include a laser and mayproduce a laser beam. A beam of light emitted by the exposure source maybe projected onto one or more mirrors each attached to a positioningdevice so that the light may be directed to a desired target location byindependently positioning each mirror along one or more axes. In somecases, a positioning device may be an element such as a galvanometer,which may be operated to rotate a mirror coupled to the galvanometerabout an axis. As one example, the exposure source 210 may comprise alaser and a galvanometer operable to direct light from the laser tovarious positions along the “Y” axis.

In some embodiments, the exposure source 210 may further be capable ofexposing both a range of the “Y” axis, which may be a subset of the full“Y” axis, and a range of the “X” axis which may be a subset of the full“X” axis. An example of one such exposure source may be multiple rows oflinear LED “bars,” or a micromirror-based digital projector, or “DLP”system. A further example of one such exposure source may be an LCDdisplay (e.g., a backlit LCD display).

In any of the above embodiments, it may be favorable to control variouscharacteristics of a light beam produced by the exposure source 210,such as spot size and/or cross-sectional shape within a build region(e.g., a focal plane within the build region), so that saidcharacteristics are as consistent as possible across the build region.Any one or more lenses may be employed to direct and/or controlcharacteristics of a light beam to achieve such consistency, such as butnot limited to one or more aspheric lenses, spherical lenses, concavelenses, convex lenses, F theta lenses, telecentric lenses, flat fieldlenses, curved field lenses, a Gradient Index Lens (GRIN Lens), orcombinations thereof. It will be appreciated that, in practice, any“lens” referred to herein may be implemented as multiple discretecomponents and as such any disclosure relating to a “lens” should not beconstrued as being limited to a single optical component.

According to some embodiments, an exposure module such as exposuremodule 109 shown in FIGS. 1A-1D and/or exposure module 209 shown in FIG.2A may comprise a light source that may be directed to desired locationsalong the Y axis by directing light onto a suitable lens. FIGS. 7A-7Bdepict two illustrative approaches to directing light within an exposuremodule in this manner.

In the example of FIG. 7A, exposure module 700 comprises a focused beamof actinic radiation 702 generated by a source 701, such as a lasermodule. Source 701 may include, and/or the emitted beam 702 may passthrough, various optical elements including one or more lenses, in orderto focus, modify the convergence of the slow and fast axis, or otherwisemodify one or more aspects of the beam 702.

In the example of FIG. 7A, one or more filtering elements 712 areincluded within the optical path and are configured to further modifythe beam 702. For instance, filtering elements 712 may include one ormore spatial filters configured to reduce population-level spatialvariability or noise in the beam profile, including higher order modesthat may be generated from the source 701, in order to produce a moreconsistent beam profile, typically with an intensity profile having anapproximately Gaussian distribution. The use of such spatial filters mayimprove the surface finish and/or reduce the minimum feature sizes ofparts produced from liquid photopolymer to which the light is directedwithin a stereolithographic device.

In the example of FIG. 7A, the beam 702 may be deflected by mirror 703,wherein the mirror 703 may be moved by a positioning device 704, such asa galvanometer to direct the light to a desired target. In particular,the positioning device 704 may be configured to deflect the beam 702 toa desired point along the “Y” axis by arbitrarily positioning the mirror703. Thus, the beam 702 may be specifically directed at an arbitrarypoint at an arbitrary time, rather than depending on a periodic motionof the deflection device 703 to reach the necessary angle or position.This configuration differs from positioning devices such as polygonalmirrors, which scan across an axis without the control afforded by apositioning device such as a galvanometer.

According to some embodiments, during operation of the exposure module700 the source 701 may be activated, thereby producing beam 702 and beam705 after reflection by the mirror 703. The mirror 703 may subsequentlybe rotated or otherwise moved by the positioning device 704 to directthe beam across a range of angles through angle 711 and thereby endingwith beam 706. During this process of exposing the build region througha range of different angles of the mirror 703, the beam exposes a linearsegment 710 along a build axis (e.g., the Y axis of the exposure source210 shown in FIG. 2A, or the Y axis of the exposure source 110 shown inFIGS. 1A-1D).

Depending on the angle of the beam 702 after deflection by the mirror703, the beam 705 may be travelling perpendicularly to the build planeor may be travelling 706 at an angle to the build plane. An angled path706 may cause certain undesirable optical artifacts, however, that maydegrade the accuracy of layers formed. In some embodiments, this may beaddressed by the use of a lens element 708, or compound elements, so asto cause beams incident onto the lens at an angle 706 to exit from thelens 708 in a perpendicular direction 709 to the lens 708, therebyachieving telecentric illumination of the build area. While the lens 708is depicted in the example of FIG. 7A as a single lens, it will beappreciated that any suitable combination of one or more lenses that areshaped to ensure that existing beams 709 form parallel beam paths may beemployed. In some embodiments, lens 708 (or another suitable combinationof one or more lenses) may produce light beams with a spot size andshape that is consistent across the entire print area (sometimesreferred to as a “flat field”).

Such an arrangement may produce a predictable photopolymer curingprofile in every location (or the majority of locations) in the buildarea, which may improve overall part quality including part precisionand dimensional accuracy. In some embodiments, the exposure module 700may be configured to create a flat field illuminating an entire focalplane within the build area using parallel beam paths perpendicular tothe focal plane (sometimes called a “telecentric flat field”).

As used herein, telecentric illumination refers at least to an opticalsystem configured to produce substantially parallel light beams andwhich intersect substantially normal to the image plane. Using a sourceof actinic radiation that is telecentric at all points of the print areamay have various benefits to the print process. One such benefit ishaving a known and consistent angle of penetration. This may provide foreach pixel or vector of exposure to occur from the same direction. Thismay improve dimensional accuracy and consistency across the print area.Some conventional devices may calibrate and adjust a source of actinicradiation using software such that light so produced may approachconsistency across all points of the build area, but with this approachthe adjusted beam may nonetheless still be inconsistently shaped and/ordivergent. As a result, conventional systems may be more sensitive tochanges in distance between optical system and print area due tomanufacturing tolerances.

In some embodiments, telecentric illumination may be produced via areflective surface such as a mirror rather than, or in addition to, alens. FIG. 7B provides an example of such an approach and depicts anexposure module 750 in which light from a light source is deflected by aparabolic mirror 714. In some embodiments, a parabolic reflector 714 maybe shaped and mounted such that the focal point of the parabola islocated in close proximity to the rotating deflecting mirror 703. Theshape of a parabolic reflector 714 may be determined in various ways,including via the parabolic formula y²=4 f x where x and y represent thecurve of the reflector in a 2D plane and f the focal length of theparabola, such as the distance between the midpoint of the reflectorcurve and the position of the deflecting mirror 703.

According to some embodiments, one advantage of the parabolic reflectingmirror in the example of FIG. 7B is that the mirror may produce both aconstant spot size and shape across a print plane (e.g., a “flatfield”), and/or may produce a consistent perpendicular beam orientationwith respect to the build plane, using a single optical element.

In some embodiments, the parabolic mirror 714 may be configured to havethe illustrated parabolic cross-sectional shape through multipledifferent cross-sections of the mirror. For instance, the mirror may berotationally symmetric about a Z-axis, with each X-Y cross-sectionthrough the mirror having the form shown in FIG. 7B. In such approaches,multiple deflecting mirror 703 and positioning devices 704 may bearranged to direct light onto any desired position on the interiorsurface of such a mirror. For example, two pairs of multiple deflectingmirrors 703 and positioning devices 704 may be arranged to deflect lightonto a desired position on the mirror along an X-axis (using onemirror/device pair) and along a Y-axis (using the other mirror/devicepair). As a result, light may be directed upwards toward a print planeafter being incident on any desired X-Y position on the parabolicmirror.

In the examples of FIG. 7A-7B, the depicted and discussed mirrors andlenses may be formed of any suitable material, such as glass, variousoptical grade plastics, and/or other materials.

As discussed above, an exposure module such as exposure module 109 shownin FIGS. 1A-1D and/or exposure module 209 shown in FIG. 2A may bedirected to portions of a build region to cure liquid photopolymer inthose portions (e.g., by controlling the positioning device 704 in theexample of FIGS. 7A-7B). In some embodiments, the exposure module maycure a layer of photopolymer in a series of scan lines running from oneside of the build region to the other, wherein the light source of theexposure module is activated and deactivated so that when the lightsource is directed to a region of photopolymer that is to be cured, thelight source is activated, and deactivated otherwise. FIG. 11 depicts anillustrative example of curing a layer with this technique.

In the example of FIG. 11, a layer is to be formed with shape 1110 (theexample of FIG. 11 depicts a build region from above or below). While anexposure source could be directed only to those parts of the buildregion necessary to cure the layer 1110, in the example of FIG. 11, theexposure source is scanned across the build region such that it traces apath 1120 within the build region. In FIG. 11, the portions of the path1120 for which the exposure source is deactivated (not curing thephotopolymer) are shown in dashed lines, whereas portions of the path1120 for which the exposure source is activated (curing thephotopolymer) are shown in solid lines.

In some embodiments, path 1120 may be produced by an exposure sourcearranged within a moveable stage that is configured to move at aconstant speed across the build region whilst the light is directed backand forth along a single axis. For instance, exposure module 109 shownin FIGS. 1A-1D and described above may be directed to produce a path1120 by sweeping the direction of the light produced by the exposuremodule back and forth along the Y axis whilst the exposure module movesunder the build region at a constant speed. In some embodiments, theexposure module may move at one speed while activated, but may move at adifferent speed (e.g., faster) when deactivated. Likewise, in someembodiments, the speed of the projected spot of light may vary, movingfaster across empty regions and/or across regions for which preciseexposure control is not necessary, such as the interior of a large curedregion within a layer.

In some embodiments, the exposure module may contain a light source andlens and/or mirror configuration as shown in FIGS. 7A-7B and describedabove. For instance, path 1120 may be produced by exposure module 700disposed within system 100, where only control of the positioning device704 combined with the motion of the exposure module is necessary toproduce the illustrated path 1120. A stereolithographic device producinglight by directing light along only one single axis from a moveablestage may have an advantage that it is easier to configure (e.g.,program) compared with a stereolithographic device that controls lightalong multiple axes simultaneously to form layers of material.

As discussed above, in some embodiments a rotating polygonal mirror orsimilar mechanism can be used to create beam steering in the fast axis.In many cases, however, it may be preferable to use a galvanometer (alsoas described above) as a steering mirror. As can be seen in the exampleof FIG. 11, a printed layer may not typically extend over the full printarea. As a result, a mechanism such as a galvanometer, which allows fordiscrete scanning lengths, can improve print time when compared with arotating mirror which always causes the full range of positions alongthe fast axis to be scanned. Additionally, it may be favorable for printspeed to vary the velocity of the steered beam between cured and uncuredsegments, in the event of a sparsely spaced printed layer. This can beachieved with a galvanometer, but not with a constant speed spinningpolygonal mirror.

While the above discussion of techniques of producing and directinglight using an exposure module have been described in the context of astereolithography apparatus, it will be appreciated that such techniquesmay also be applied in other types of additive fabrication device. Forinstance, other additive fabrication devices in which a light beam isdirected onto a material, such as selective laser sintering (SLS)devices, may incorporate an exposure module configured to produce a flatfield or a telecentric flat field as described above. For example,exposure module 700 or 750 may be employed in an SLS device and light soproduced directed to melt or otherwise consolidate a powdered material.

In some embodiments, a thin film such as film 103 shown in the exampleof FIGS. 1A-1D may comprise multiple layers of material, such asmultiple layers of films and/or films comprising different materials.FIG. 6 depicts one example of such a film.

In the example of FIG. 6, a tank 600 is depicted that includes rigidsides 104, tensioning device 107 and a multi-component film comprising afirst film 603A forming the bottom of the tank 104 and a second film603B located between the first film 603A and the exposure module 109.

In some embodiments, film 603A may comprise a first material that iscompatible with and/or not appreciably degraded by contact with expectedconstituents in desired photopolymer materials. Advantageously, thefirst film 603A may comprise one or more materials selected to berelatively impermeable to substances within the desired photopolymerresin. It may further be advantageous to select a material to be incontact with the photopolymer resin such that the liquid photopolymerand the selected material possess a high degree of wettability withrespect to each other. In particular, it may be desirable to form thinfilms of liquid photopolymer resin having a consistent thickness againstthe surface of the film 103 material for subsequent exposure to actinicradiation. To the extent that material possesses a low partial wetting,such a layer may tend to form beads or otherwise tend to cohere ratherthan to spread readily across the surface of the material into asubstantially uniform thin layer. FEP, Teflon AF, and other such“non-stick” surfaces, typically comprise surfaces with low surfaceenergies, thus providing poorly wetted surfaces with regards to liquidphotopolymer resin. While this low surface energy may be advantageousfor the separation of cured photopolymer resin, it is undesirable withregards to the formation of thin films of liquid photopolymer resin. Theinventors have determined that poly(4-methyl-1-pentene), or PMP, incontrast, is substantially more wettable with respect to a wide range ofliquid photopolymer resins than FEP, such that thin films ofphotopolymer resin may more reliably be formed against a first materialformed of PMP, despite the fact that PMP possesses excellentseparability with respect to cured photopolymer. In particular, theinventors have found that a layer of PMP provides for an effective film103 for use with a wide range of photopolymer resins.

In some embodiments, film 603A comprises, or is comprised of, PMP. Insome embodiments, film 603A may have a thickness of greater than orequal to 0.001″, 0.002″, 0.004″. In some embodiments, film 603A may havea thickness of less than or equal 0.015″, 0.010″, 0.008″. Any suitablecombinations of the above-referenced ranges are also possible (e.g., athickness of greater or equal to 0.002″ and less than or equal to0.008″, etc.).

While film 603A may be formed from one or more materials (such as PMP)that both provide good separability with respect to cured photopolymerand are also compatible with a wide range of photopolymer materials,such materials may however lack other desirable properties, such asmechanical properties suitable for rolling or sliding interactions withthe exposure module 109. The second film 603B may, in some embodiments,be configured to provide such properties so that the combinedmulti-component film exhibits each of the above desirable properties.

According to some embodiments, film 603B may comprise Polyethyleneterephthalate (PET). According to some embodiments, film 603B maycomprise an aliphatic thermoplastic polyurethane or TPU. According tosome embodiments, film 603B may comprise polystyrene. For instance, film603B may comprise, or may be comprised of, a film of optically clearpolystyrene.

Irrespective of the material composition of film 603B, in someembodiments, film 603B may have a thickness of greater than or equal to0.001″, 0.002″, 0.004″. In some embodiments, film 603B may have athickness of less than or equal 0.015″, 0.010″, 0.008″. Any suitablecombinations of the above-referenced ranges are also possible (e.g., athickness of greater or equal to 0.002″ and less than or equal to0.008″, etc.).

In some embodiments, the second film 603B need not be selected tooptimize for exposure to photopolymer resin constituents, as deleteriousconstituents may have only limited permeation through the first film603A. Instead, the second film 603B may be selected from variousmaterials with other advantageous properties, such as comparatively highdegrees of tensile strength and/or resistance to creep or otherdeformation when placed under tension. In some embodiments, the materialused to form the second film 603B may be flexible, but comparativelyinelastic (e.g., a thin material with both a comparatively high yieldstrain and Young's modulus).

In some embodiments, second film 603B may be in periodic contact withvarious mechanical devices, such as roller elements 111, which maycontact second film 603B and/or exert forces against it while in motion.Accordingly, the second film 603B may be advantageously selected frommaterials with suitable mechanical properties for such repeated contact,such that a lower wear may be achieved. In certain embodiments, suchproperties may also include superior resistance to abrasion andpuncture, comparatively low friction and/or a comparatively high degreeof lubricity. In may further be advantageous to select a material withsubstantial elasticity, such that the second film 603B may be resistantto punctures or other failure modes where excess force is applied.Moreover, the presence of a second film may reduce in any damage orother impact caused by the release of photopolymer resin upon a failureof the integrity of the first film 603A.

In some embodiments, it may be advantageous to add a coating to a filmin contact with mechanical devices such as roller elements 111. Thiscoating may preferably be transparent to actinic radiation of one ormore relevant wavelengths. This coating may be comprised of any numberof materials compatible with the film, of suitable transparency, andwith an increased wear resistance or hardness relative to the film. Insome embodiments the coating may be comprised of an acrylic or urethanebased coating. It may be advantageous for such a coating material tohave properties favorable to wear resistance or a high hardness.

In some embodiments, the first film 603A and second film 603B may beseparate films and may form at least a partial gap 604 located betweenthe film 603A and film 603B, as shown in FIG. 6. In some embodiments,gap 604 may be produced by mounting films 603A and 603B with an offsetbetween the films.

In some embodiments, film 603A and/or film 603B may be mounted undertension. In such cases, the tension forces applied to the films may tendto cause the films to come into contact, potentially closing off any gap604 during the application of tension. In some embodiments, the transitof the exposure module 109 and contact between the exposure module 109and film 603B may cause film 603B to move upwards, into greater contact605 with film 603A, thereby causing the gap 604 to close at and/oraround the point of contact.

The inventors have observed that the presence of a gap 604 may beadvantageous where film 603B may have a lower oxygen permeability ascompared to film 603A. Without intending to be limited to a specifictheory, the inventors believe that the gap 604 may allow for theimproved transport of oxygen through the film 603A, thus causing certaininhibition effects with respect to photopolymerization. Such a gap 604may therefore allow for the selection of a material for film 603B withreduced consideration for the oxygen permeability of the film 603B.Moreover, such an improvement may allow for relatively thicker films tobe utilized for films 603A and 603B, while maintaining acceptabledegrees of oxygen permeability.

In some embodiments, gap 604 may be produced such that the gap ismaintained under the influence of film tensioning. For example, spacingelements may be arranged between film 603A and 603B so as to enforce agap 604 between the films, at least at or close to the spacers, even ifthe films are under tension that would otherwise cause them to besubstantially in contact with one another. Such spacing elements may beregularly located along the interface between the films. Alternatively,a spacing element (not shown) may be moved between the films so that agap 604 may be produced and moved to desired locations. In some cases,the gap produced may not be positioned directly at the position of thespacer, and may effectively move in advance of and/or following themotion of the spacing element.

In some embodiments, film 603A and/or film 603B may include non-planarsurface features, such as channels, relief features, and/or othertextures, such that a plurality of partial gaps 604 may be formed evenwhen the films 603A and 603B are substantially in contact with oneanother. In some embodiments, such gaps 604 may be ensured by the use of“matte” or otherwise micropatterned films. In some embodiments thismatte or frosted coating may be advantageous for a number of reasons.Frosted films may be less prone to suction forces common between twosmooth films. Frosted films may also be useful in other applications asthey may act as a filter to reduce sensitivity to differing laser spotsizes.

The inventors have observed that the presence of a gap 604 may beadvantageous, particularly where film 603B may have a lower oxygenpermeability as compared to film 603A. Without intending to be limitedto a specific theory, the inventors believe that the gap 604 may allowfor the improved transport of oxygen through the film 603A, thus causingcertain inhibition effects with respect to photopolymerization. Such agap 604 may therefore allow for the selection of a material for film603B with reduced consideration for the oxygen permeability of the film603B. Moreover, such an improvement may allow for relatively thickerfilms to be utilized for films 603A and 603B, while maintainingacceptable degrees of oxygen permeability.

In applications utilizing thin films, such as film 103, 603A, and/or603B, shown in FIGS. 1A-1D and FIG. 6 and discussed above, it may bedesirable for the film to be mounted onto a structure, such as the tank104 in the example of FIGS. 1A-1D. In many cases, it is also desirablefor said film to be mounted under tension, or for tension to otherwisebe applied to the film at various points of operation.

Conventional films may, however, exhibit a number of typically unwantedbehaviors, such as gradual deformation in the direction of tensionforces, sometimes known as “creep.” For example, as shown in FIG. 3, afilm 301 placed under tension along a single axis 302 (uniaxial tension)may deform, or “waist” 303, in such a way as to cause the dimension ofthe film to decrease in an axis orthogonal to the axis of the tensionforce in question. This deformation may, particularly in thin films,result in substantial waves or other distortions in the film, where anotherwise flat film forms non-planar deformations 304, or “wrinkles.”And, in stereolithographic applications, such non-planar defects in afilm may be especially problematic where such film is intended to be aflat reference surface against which photopolymer material may besolidified, such as discussed above.

Some stereolithographic devices using tensioned films attempt to addresssuch wrinkling behavior by attempting to place a given film underuniform tension in multiple axes, such as shown in FIG. 4. Thisapproach, referred to herein as a “drumhead” mount, typically requiresthe film 401 to be secured to a structure on the sides of the film 401,such as by pins 403 or other mounting techniques, and a primary sourceof tension loaded onto the film 401 along an axis 402. The objective isto both tension the film in one axis 402 while preventing the film fromcontracting (or waisting) in a second axis 404. Even using such asystem, however, an unequal application of tension forces may result inthe same manner of deformations caused by uniaxial tensioning. As aresult, conventional mounting systems for tensioned films may requireundesirable strength, complication, and/or adjustment, in order toprovide for a film with both the desired degree of tension and flatness.Moreover, the use of multiple films, such as 603A and 603B, formed ofdiffering materials, and with potentially inconsistent lengths due tomanufacturing tolerances, adds additional complexities to theabove-described challenges with mounting thin films.

The inventors have recognized and appreciated improved techniques forproviding film tensioning. FIG. 8 depicts an illustrative adjustabletension system 800, according to some embodiments. In the example ofFIG. 8, a tank is adjustably tensioned by means of a tensioning device801, which is external to the tank and associated with astereolithographic device in which the tank is installed. Thisconfiguration may allow the tank to be removable from the device,wherein the illustrated system 800 includes tensioning device 801 aspart of the device with the remaining depicted components being elementsof the tank. As shown in FIG. 8, the tank 800 may comprise a film 803Aand film 803B, forming a film system 803, such as described above inrelation to films 603A and 603B shown in FIG. 6. The films 803A and 803Bmay be mounted onto or around a distribution arm 802 at points 804A and804B, respectively. In some embodiments, the distribution arm 802 may bea rotationally unconstrained shaft element.

Distribution arm 802 may be mounted via axis 805 onto a tensioningarmature 806, which is configured to be rotatable about axis 807 withina tank side structure 808. Tensioning armature 806 comprises a couplingarm 809 which extends outward such that it may be at least partiallycaptured by a tensioning device 801, associated with astereolithographic device. The films 803A and 803B may be mounted on anopposing side tank structure 808 at a static mounting point 810.Alternatively, both sides of the tank structure 808 may include dynamicmounting elements. When not inserted into a stereolithographic device orotherwise engaged via the coupling arm 809, the tensioning armature 806may adopt a relaxed position 811, resulting in a comparatively lowertension placed on films 803A and 803B. In some embodiments, the staticmounting point 810 may be attached to a rotational axis in a similarmanner to the coupling of elements 802 to axis 807. Incorporating thisadditional rotational axis coupled to the static mounting point mayensure the film maintains a desirable planar surface during tensioning,since the mounting points 802 and 810 may move to maintain the films ina substantially parallel arrangement.

When the tensioning mechanism 801 of the stereolithographic device iscoupled to the coupling arm 809 of the removable tank, and displacedalong axis 812, the tensioning armature 806 may be caused to rotatealong axis 807, thus displacing the distribution arm 802 in the samedirection along axis 812. As will be appreciated, such a motion of thedistribution arm 802 away from the opposing mounting 810, or away fromthe opposing dynamic mounting elements in the case of two or moredynamic mounting elements, may result in an increase in tension alongfilms 803A and 803B and, potentially, a degree of extension or otherdeformation of said films in response to the tension forces.

Since, in the example of FIG. 8, the distribution arm 802 may rotateabout axis 805, the amount of tension applied to films 803A and 803B asa result of motion along axis 812 need not be constant.

According to some embodiments, distribution arm 802 may form awhippletree (also known as a whiffletree) linkage, distributing forcesapplied via the axis 805 between the films 803A and 803B, attached atpoints 804A and 804B on the distribution arm 802. According to someembodiments, a whippletree or whiffletree linkage may refer to a rigidbody able to apply two or more forces to two or more different points.

While the distances between the axis 805 and the attachment points 804Aand 804B are shown to be symmetric in the example of FIG. 8, otherembodiments may locate axis 805 such that a desired differential ratioof tension forces may, as a result of varying mechanical advantages onopposing sides of the whippletree linkage, be applied to films 803A and803B for a given displacement along axis 812. Such a differential may beparticularly advantageous where films 803A and 803B are formed ofmaterials with dissimilar responses to tension forces, such as differingelastic constants.

In some embodiments, instead of independently attaching films 803A and803B to the distribution arm 802, the films 803A and 803B may be bondedtogether at one end such that they are looped around the distributionarm 802 which may be a rotationally unconstrained shaft element. Thisconfiguration may, in at least some cases, allow the unconstrained shaftelement to compensate for small differences in the slack of each filmdue to manufacturing tolerances, minor imperfections, and/or differingreactions to repeated mechanical forces such as different degrees ofcreep. In some embodiments, the films 803A and 803B may be joined orcrimped together at one or more edges while allowing at least one freeedge to provide oxygen permeability. The films may be adhered orattached at one end and at the crimp location by any number of methodsincluding pins, adhesives, lamination, a crown piece, etc.

Another aspect of the present invention allows for the uniaxialtensioning of a film without resulting “wrinkle”-type deformations orsimilar non-planar features forming. An illustrative embodiment of thisaspect is depicted in FIG. 5. As shown, a thin film 501 may be mountedalong an axis 504 under tension along axis 502. In this illustrativeembodiment, mounting pins 503 may be used to mount the film 501 undertension along the axis 502.

In contrast to the above-described mounting examples shown in FIGS. 3and 4, however, mounting pins 503 are oriented non-linearly along axis504. In particular, the position of the each mounting pin of mountingpins 503 may depend upon the distance of the pin 503 from the midlineaxis of the film 502a, such that pins, such as 503 b, further from themidline axis 502 a cause the film 501 to extend a greater distance 502 bthan pins, such as 503 a, closed to the midline axis 502 a. By mountingthe film 501 under such a variable geometry, the inventors have foundthat both planar and non-planar deformations may be substantiallyreduced.

Without intending to be limited to a specific theory, the inventors havefurther observed that at least one effective arrangement of pins 503 maycorrespond with the pattern of tensile forces applied by the film 501during non-planar deformation. As shown in FIG. 3, for example, a film301 may be tensioned along on axis 302. After a period of time, whichmay depend upon the amount of tension, geometry of the film, and/or thematerials of the film, deformations 303 and 304 may tend to develop andreach a comparatively stable configuration. Upon measurement, theinventors have appreciated that such a deformation process may result inan uneven application of tensile forces in axis 302 along the mountededge of the film 301 running perpendicular to said axis. Accordingly,tensile forces exerted at each point along the mounted edge of the film301 may be measured in various ways, such as by instrumenting individualmounting pins. Pins 503 may then be offset based upon the distance fromthe midline axis of the film 502 a such that the amount of force exertedby the pin 503 is approximately the same as the amount of forcepreviously experimentally measured following deformation.

The opposing side of the film 501 (not pictured) may be mounted in anysuitable way, such as, but not limited to, mounted using variably offsetpins 503. Alternatively, or in addition, experimentally determinedindividual force measurements may be fit to a curve in order to generatesuitable force calculations for arbitrary distances from the midlineaxis 502 a. In some embodiments, the inventors have found curves ofbetween 4th and 6th order to best approximate experimentally measuredforces as a function of distance from the midline axis of the film 502a.

In some embodiments, mounting tensile forces may be applied against thefilm 501 in a more continuous manner. As one example, film 501 may beconventionally mounted under tension, but where a deflecting elementcontacts the film 501 such that the film 501 is forced to deflect alongthe profile of the deflecting element, the profile taking theapproximate shape of the curve fit as described above. In the embodimentillustrated in FIG. 8, the mounting edges 804A and 804B along which thefilm is attached may be linear, or may instead be non-linear and followthe curve or profile described above, thus forming both an attachingpoint and a deflecting element, based upon the curve of the mountingedge 804A or 804B. In some embodiments, the distribution arm 802 mayhave the shape of the curve or profile described above such that thefilms wrapped around the distribution arm 802 are subject to the samemounting profile.

As shown in the example of FIG. 8, tension forces applied to the films804A and 804B may be applied via displacement of the tensioning device801 along axis 812. As discussed above, in the example of FIG. 8 thetank structure may be removable with the tensioning device 801 locatedoutside of the replaceable structure. In some embodiments, tensioningdevice 801 may include any one or more conventional sources of tensionforces, such as those produced by extension or torsion springs,potentially requiring user involvement to manually provide initialloading of the system. In some embodiments, tensioning device 801 maycomprise a source of linear force, such as a hydraulic cylinder and/orother form of linear actuator. In some embodiments, rotational force maybe converted into a linear displacement of the device 801 via means suchas rack and pinion mechanisms, thus causing tension forces to be appliedto the system.

In some embodiments, it may be advantageous for the tensioning device801 to apply tension against the film only during the operation of themachine, such that tension is removed or otherwise reduced when themachine is not in use. Such detensioning may be helpful in preservingthe working lifetime of the film and/or may allow for a removable filmto be removed following a power loss or other failure of the devicewithout requiring removal of the film while under tension.

In some embodiments, a stereolithographic device that includes dynamictensioning system 800 may adjust the amount of displacement of thetensioning device 801 along the axis 812 during or between cycles ofoperation. In general, the amount of force applied to films 803A and803B by a given amount of displacement along axis 812 may be dependentupon various physical properties of the film, including but not limitedto a spring constant k. A comparatively inelastic material may have ahigh k value, such that a comparatively small amount of displacement (x)along the axis 812 results in a comparatively large amount of tensileforce (F), as may be appreciated by an application of Hooke's Law(F=kx). As a result, the displacement applied may be selected based on adesired force according to Hooke's law.

In some embodiments, various physical properties of the resin tankassembly may affect the force applied across the films 803A and 803B fora given amount of displacement along axis 812, such as the ability ofthe assembly or one or more other components to resist torsional,translational, or rotational forces. The extent to which thecomponent(s) resist forces may depend on various factors including butnot limited to the type and thickness of material used and the directionof forces applied to achieve tensioning. In some embodiments, thetensioning device 801 and/or the tensioning armature 806 including thecoupling arm 809 may be configured to resist forces applied across thefilms 803A and 803B in order to achieve a desired tensioning. By way ofexample, instead of a rotational axis 807 as in the example of FIG. 8,the tensioning armature may instead be configured such that it travelsalong one or more channels or translational elements in the tank side808 in order to limit the upwards rotation of the film as the device istensioned. In each case, the rotational axis or the translationalelements, the tank side may serve the function of counteractingtorsional forces involved in this tensioning mechanism. In anotherembodiment there may be no rotational or translational channel elementsand instead the coupling arm and the tensioning device could bestructured with additional reinforcing structures to combat the effectof a rotational force without depending on the resistance provided bythe tank side 808. In said embodiment the reinforcing structures couldbe additional heel or toe features on the connection points that work inconjunction to resist rotational moments in combination with each other.For example, adding a heel or toe feature to either connection point mayadvantageously distribute forces such that rotational movement may belimited.

In some embodiments, the application of tension to films 803A and 803Bmay cause a gradual deformation of the films, including stretching orelongating in response to the tension. Such distortions may cause areduction in the amount of tension generated by additional displacementof the tensioning device 801 along the axis 812. In some embodiments,these effects may be managed via the application of a passive tensioner,such as a counter-spring. For instance, tensioning device 801 may beattached to an extension spring extending along axis 812, such that the“resting” state of the spring causes a force to be applied to thetensioning device 801 along axis 812 away from the tank structure 808.In some embodiments, however, it may be more advantageous to utilizemore active tensioning means, such that additional control over theprocess may be provided. For example, it may be advantageous to vary theamount of tensile forces applied to the film in order to optimize forvarious process parameters.

In some embodiments, a stereolithographic device that includes dynamictensioning system 800 may include one or more components configured tomeasure the amount of tensile force applied by the tensioning device801, such as via strain measurements or other sensing techniques. Suchmeasurements may, in some cases, be utilized by various control means toprovide a form of “closed loop” control over the amount of tensionapplied via adjusting the positioning of the tensioning device 801.

An illustrative tensioning device 900 is depicted in FIG. 9, accordingto some embodiments. As shown in the example of FIG. 9, an actuator 902,such as a linear motor or rotational motor combined with a rack andpinion type gearing, is configured to cause displacement of a drivingrod 903 along axis 904. As a result, the pin 909 moves within a slot908, causing motion of the tensioning plate 910 along axis 912. Thecoupling arm 901 may thereby be moved with varying tension. According tosome embodiments, tensioning device 900 may be used in the example ofFIG. 8 where the coupling arm 901 is the tensioning device 801 thatcouples to the coupling arm 809.

Actuator 902 may be instrumented in various ways, including by use of anencoder to provide position information for driving rod 903 and/orstrain or stress gauges to measure the amount of force applied viadriving rod 903. In the example of FIG. 9, driving rod 903 is coupled toan extension-type spring 905, having a spring constant K along axis 904.Spring 905 is attached, via driving rod 906, to a first coupling plate907. Coupling plate 907 includes a slot 908. Tensioning plate 910includes a pin 909 which extends through slot 908 forming a cam-typelinkage 908-909, such that motion of coupling plate 907 along axis 904causes motion of tensioning plate 910 along axis 912. During operation,actuator 902 generates a linear displacement along axis 904 which,ultimately, results in tension forces within the film system 803. Thisdisplacement, however, is transmitted and modified via extension spring905. In particular, extension spring 905 may be advantageously selectedto have a spring constant substantially lower than the effective springconstant of a film system to which the tensioning device 900 is coupled(e.g., film system 803 in the example of FIG. 8). As may be understood,coupling plate 907 and slot 908 may be oriented in any manner such thatthe slot 908 affects the motion of pin 909 to provide the desired motionto the tensioning plate.

As discussed above, various materials used for the formation of filmsmay be comparatively inelastic, indicating that they provide acomparatively high spring constant k, in terms of Hooke's Law. As aresult, the tensile force applied to the film may be sensitive to smallchanges in the amount of displacement of driving rod 903 along axis 904,requiring a comparatively high level of precision in the positioning ofdriving rod 903. By use of a spring 905 having a spring constant smallerthan the comparatively high spring constant of the film or system offilms, however, a wider range of displacement of driving rod 903 alongaxis 904 may be acceptable, allowing for higher precision in the amountof tension forces generated for a given displacement precision. Inparticular, a displacement of driving rod 903 along axis 904 away fromthe tensioning plate 910 may result in a significantly smallerdisplacement of driving rod 906 along axis 904 based upon the ratio ofeffective spring constants. Displacement of driving rod 906 subsequentlycauses coupling plate 907 is be displaced along axis 904. Due to linkageformed by slot 908 and pin 909, the displacement of the coupling plate907 along axis 907 causes a displacement of tension plate 910 along axis912.

Depending on the geometry of slot 908, however, the tension plate 908may not be displaced the same distance along axis 912 as coupling plate907 along axis 904. As an example, the slope of a linear path 908 maydefine a ratio of motion between tension plate 908 and coupling plate907. In some embodiments, the inventors have found it advantageous forthe path 908 to be non-linear or multi-linear (i.e., composed of linearsegments with varying slope), such that the ratio of displacementlengths between the coupling plate 907 and tension plate 908 depends inpart upon the location of the tension plate 908. In particular, theinventors have noted that progressive deformations in the film system803, such as creep, may result in a gradual offset in the requiredposition of tensioning plate 910 in order to effect an equivalent amountof tension force in film system 803. As one example, an illustrativefilm system 803 may, over time, distort, increasing in length along axis912. As a result, a tensioning plate 912 applying a tension force to thefilm system may need to be positioned significantly further away fromthe tank along axis 912, thus requiring in turn that the coupling plate907 be positioned significantly closer to the actuator along axis 904.Such a positioning of the actuator, however, may result in lessextension in spring 905, and thus a lower total force applied to thefilm system. In contrast, embodiments utilizing a non-linear path 908,the path 908 may be curved such that the transmission ratio between thetensioning plate 910 and coupling plate 907 is increased the furtheraway from the tank the tensioning plate 910 is located along axis 912.Accordingly, less displacement along axis 904 may be required to achievethe required displacement along axis 912.

As discussed above in relation to FIGS. 1A-1D, layers of material may beformed on a build platform that is configured to move toward and awayfrom the build region. Since the build platform must be placed precisely(e.g., to within several microns) for each layer to be formed with adesired thickness, it is important that the stereolithographic device isable to track the vertical position (the Z axis position) of the buildplatform.

Conventional stereolithography devices may use positioning devices suchas linear actuators and rack and pinion-type transmissions. In somecases, a source of rotational motion, such as a stepper motor, mayrotate a threaded rod extending along an axis, sometimes known as az-axis “screw.” The build platform may be mounted, such as by use of acaptive nut or similar hardware, such that the rotation of the threadedrod causes the captured nut and platform to be forced up or down alongthe axis in proportion to the rotations of the threaded rod. The motionof the build platform and threaded rod may, however, have limitedinstrumentation and as such it is conventionally assumed, for thepurposes of motion control and planning, that the stepper motor hasunlimited torque and does not experience any “missed steps,” regardlessof the opposing forces exerted onto the motor through the threaded rod.Such assumptions may not always be valid, however.

While some approaches utilize various indirect measurements in order todetect the resistance to motion, or force, applied against the buildplatform (e.g., as set forth in U.S. application Ser. No. 15/623,055,titled “Position Detection Techniques for Additive Fabrication andRelated Systems and Methods,” filed on Jun. 27, 2017), it may still notbe possible to obtain the desired accuracy in force measurements.Moreover, such measurements may typically only be obtained near or atthe torque limits of the system, thus increasing the chances of failureor unwanted wear. These problems, and others, may be addressed by theuse of embodiments of the invention, such as illustrated in FIG. 10,including in-line force sensing within a linear motion system.

As shown in FIG. 10, a source of motion 1001 may be coupled to a rod1005 for conveying linear motion. In the example of FIG. 10, the sourceof motion 1001 is a source of rotational motion (e.g., a stepper motor),conveyed along a rod 1005 and converted from rotational motion intolinear motion. In other implementations, however, the source of motioncould instead directly introduce linear motion along rod 1005. In eithercase, actuation of the motion source 1001 may cause a force to beapplied to the motion source 1001 along axis 1008. For instance, in theexample of FIG. 10, actuating the motion source 1001 may cause threadedrod 1005 to rotate such that a captured nut, that is a nut which may notfreely rotate, experiences a linear force along axis 1008 towards themotion source 1001. At the same time, motion source 1001 may experiencean opposing force along axis 1008, “pulling” it towards the capturednut.

Some conventional systems have dealt with such opposing forces byensuring that motion source 1001 is mounted to a substantially rigidstructure that is capable of resisting the expected forces andpreventing substantial movement of the motion source 1001.

In contrast, in some embodiments a linear motion system may insteadmount motion source 1001 such that opposing forces will tend to causemovement of the motion source 1001 at least in part proportional to themagnitude of such forces. As shown in the example of FIG. 10, forinstance, motion source 1001 is mounted using a spring-like, deformablemounting structure 1002, which is in turn mounted onto rigid structures1003. In some embodiments, the deformable mounting structure may be abracket or other structure onto which the motion source 1001 is mountedand wherein the bracket is free to move in a direction parallel to thedirection of the opposing forces described above. For instance, oneportion of the bracket may be mounted to a rigid structure and anotherportion may be attached to the motion source 1001. In such an approach,the bracket may bend around the mount of the bracket to the rigidstructure when forces are applied to the motion source 1001.

During operation, opposing forces against motion source 1001 may causethe motion source 1001 to move along axis 1008 and cause the deformablemounting structure 1002 to deflect along axis 1008 in proportion to theamount of force applied.

For example, a spring-like deformable mounting structure 1002 maycomprise, or be comprised of, a sheet of spring steel having a Hookespring constant of k along the axis 1008. An application of a force Falong this axis 1008 may thus cause the deformable mounting structure1002 to deflect approximately a distance F/k along axis 1008 as a resultof Hooke's Law. As a result, measuring the amount of this deflectionindicates the amount of force applied by the motion system. Forinstance, the measurement of deflection may be converted into ameasurement of the forces by applying Hooke's Law and/or a suitablemathematical or heuristic model describing the deflection of themounting structures 1002 in terms of the amount of force applied againstsaid structures.

The amount of deflection of the deformable mounting structure 1002 maybe measured in any suitable way, such as by detecting deflection viaoptical and/or mechanical measuring means. The amount of force appliedby the motion system may then be estimated based on this measurement. Inthe example of FIG. 10, a measurement of the amount of deflection of thedeformable mounting structure 1002 may be obtained via one or morenon-contact sensors 1006 mounted on a reference plate 1007. Thenon-contact sensors 1006 may, for instance, measure a position of thedeformable mounting structure 1002 via inductive and/or capacitivesensing.

Various forms of sensors 1006 may be utilized. One consideration is thedegree of measurement accuracy required, which is partially determinedby the expected amount of deflection. In some embodiments, the inventorshave found that compact size is a primary concern, and so have chosenspring-like mountings with comparatively high spring constants, such as1 N/um, thus requiring comparatively high precision measurements over acomparatively small distance. In some embodiments, the inventors havefound that inductive distance sensing may provide such measurements,particularly when combined with a reference target 1004 formed of analuminum inductor. In such configurations, sensors 1006 may compriseinductive coils connected to an inductance-to-digital converter, such asthe LDC1612 processor sold by Texas Instruments, in order to generate adigital signal corresponding to the distance between the sensor coils1006 and the reference target 1004.

In the example of FIG. 10, according to some embodiments, a referencetarget 1004 may be mounted onto the deformable mounting structure 1002.The reference target 1004 may be a structure that is more easily sensedby the non-contact sensors 1006 than the deformable mounting structure1002.

Various applications may be envisioned for the position detectiontechniques described above in relation to FIG. 10. For instance, theforce sensor of FIG. 10 may be coupled to a build platform such that theaxis 1008 is the Z-axis direction of motion of a build platform (e.g.,axis 106 shown in FIGS. 1A-1D for build platform 105). Thisconfiguration may allow a force being applied to the build platform tobe measured via the force sensor described above. Measuring a forceapplied to a build platform may have various beneficial applications,including measuring an extent to which a surface below the buildplatform (e.g., a container) resists downward motion of the buildplatform, an extent to which upward motion of the build platform isresisted (e.g., as a result of the build platform, or a part attached tothe build platform, adhering to a surface such as a container). Variouscalibration and error checking operations may be envisioned based onsuch measurements, examples of which are described below.

FIG. 13 depicts an illustrative sequence of measurements taken by aforce sensor that measures a force applied to a build platform,according to some embodiments. In the example of FIG. 13, graph 1300shows a measurement of force (vertical axis) taken over time (horizontalaxis). Initially in the pictured sequence, an initial layer of a part isbeing formed on a build platform or on previously formed layer of a partattached to the build platform (e.g., as in the example of FIGS. 1A-1Ddiscussed above). Subsequently, the build platform begins to pull on thepart, and thereby on the container to which the part is adhered. Thiscauses the force measurement to increase in magnitude at 1305 as thecontainer initially resists this motion. This force increases at 1310until the part separates from the container and the force quicklyreduces at 1315. While various different types of container may beutilized in stereolithography, including rigid containers with anelastic coating or container having a thin film as a bottom surface, itis expected that a spike in the force measurement would be observed ineach case, although the exact form of the force versus time measurementsmay be quite different for different container materials. As a result,irrespective of the type of container, a moment at which a partseparates from a container may be identified by identifying a spike inthe force measurement, the spike being a rapid increase and decrease inthe force measurement. Alternatively or additionally, separation may bebased on detection of the force being within a zero window (e.g., window1330).

In some embodiments, a sequence of motions of the build platform may beselected based on a time at which the part is detected to separate fromthe container. For instance, a “squish” move, which is a sequence ofmotions in which the part is positioned close to the container inpreparation for forming another layer and typically ends with a periodin which the part is held at a fixed position (a “squish wait”), may beperformed based on how long separation of the part from the container isobserved to take. A plurality of pre-baked squish moves may be stored orotherwise accessed by the printer and one of the moves selected andperformed based on a length of time between initiating separation of thepart from the container and detecting completion of said separation.

In some embodiments, various operations of a wiper in an additivefabrication device may be adapted based on measurements by a forcesensor that measures a force applied to a build platform. For instance,the applied force to separate a part from a container may depend on theviscosity or other properties of the liquid in the container. As aresult, the wiper may be operated based on the force, which mayimplicate particular properties of the liquid. For example, the wipermay operate comparatively longer when the measured force indicated theliquid is comparatively more viscous to aid in recoating. As anotherexample of adapting wiper motion based on measurements by a force sensorthat measures a force applied to a build platform, as the surface areaof a part contacting the container increases, the measured force mayalso increase due to increased adhesion. Recoating of the larger area inthe absence of a wiper may also increase because there is more space forthe liquid to flow back into after the part is moved away. As a result,the wiper may be operated to perform greater recoating when the surfacearea of contact is greater. In general, however, the speed of wiping,pauses between wiping, number of wiping cycles, squish wait may all beadapted based on the force measurement.

FIGS. 14A and 14B depict another application of force sensing using abuild platform, according to some embodiments. In the example of FIG.14A, a build platform 1410 is coupled to a force sensor such as thearrangement shown in FIG. 10. The build platform has an attached probe1415, which may comprise any vertically extending shape that is gluedon, adhered to, or otherwise attached to the build platform. In somecases, the probe 1415 may be a part fabricated onto the build platformvia typical printing techniques. The probe 1415 may or may not have aknown height.

In the example of FIGS. 14A and 14B, two rollers 1421 and 1422 of anexposure module (e.g., rollers elements 111 in stereolithographic device100 shown in FIGS. 1A-1D) are arranged below the build platform 1410such that one roller is beneath the probe 1415. The build platform maybe lowered toward the roller whilst a force applied to the buildplatform is measured. Contact between the roller and the build platformmay be identified when a force resulting from contact is measured by thebuild platform, as shown in FIG. 14B. In cases where the height of theprobe 1415 is known, a position of the roller 1422 may be determinedbased on the probe height and the position of the build platform whenthe force is detected. The force detection may be performed in numerousways, including by detecting a force above a given threshold value.Another way to detect the height of the probe is to measure the rate ofchange of the force with height. Subsequent to touching the rollers, therate of change of the force with height is essentially due to the springconstant of the machine. The height at which Hooke's law implies a zeroforce by fitting observed data to the spring constant can thereby beused to identify the height at which the build platform touched theroller.

In some embodiments, the height of the probe 1415 may not be known, butcan be used to determine the different in heights (the “bias”) betweenthe rollers 1421 and 1422 (labeled as distance 1431 in FIG. 14B). Todetermine the bias, the probe may be lowered onto each roller and aposition at which contact is identified via the force measurementsrecorded. The bias 1431 can thereby be determined as the different inthe vertical positions recorded, which is independent of the probeheight.

Additional techniques in which measurements of a force applied to thebuild platform may be applied are to detect whether a build platform hasbeen installed correctly. If a build platform is simply missing, theforces measured over time during motion of the Z axis will be differentcompared with when a build platform is installed due to the differentweights. In addition, if the build platform is installed but is movingincorrectly (e.g. is sticking in its motion, producing spikes in theforce measurements over time), this can also be identified through forcemeasurements on the Z axis.

In some embodiments, a user initiating a print with a build platformthat has material on its surface may be detected from force measurementsdue to the increased weight of the build platform. Such detection may beparticularly beneficial when the container comprises a thin film becauseof the propensity of the film to be damaged by applied force and/orinteraction with sharp edges. A stereolithographic device may providefeedback to a user in the above cases where an issue is detected thatmay cause the device to be damaged and/or for reduced quality of a printmay be expected.

FIG. 12 is a block diagram of a system suitable for practicing aspectsof the invention, according to some embodiments. While the abovedescription relates primarily to a stereolithography device and itscomponents, it will be appreciated that such a device may be programmedand/or otherwise controlled by a suitable computer system.

System 1200 illustrates a system suitable for generating instructions tocontrol an additive fabrication device to perform operations asdescribed above. For instance, instructions to operate one or more lightsources (including turning the light source on and off as discussed inrelation to FIG. 11), light directing components associated with suchlight sources (e.g., computer adjustable mirrors, such as mirrorgalvanometers), sensors, move an exposure source beneath a film surface,move a build platform, sense a position of the build platform, and/oradjust tension of a film system may be generated.

According to some embodiments, computer system 1210 may execute softwarethat generates two-dimensional layers that may each comprise sections ofthe object. Instructions may then be generated from this layer data tobe provided to an additive fabrication device, such as additivefabrication device 1220, that, when executed by the device, fabricatesthe layers and thereby fabricates the object. Such instructions may becommunicated via link 1215, which may comprise any suitable wired and/orwireless communications connection. In some embodiments, a singlehousing holds the computing device 1210 and additive fabrication device1220 such that the link 1215 is an internal link connecting two moduleswithin the housing of system 1200.

An illustrative implementation of a computer system 1500 that may beused to perform any of the techniques described above is shown in FIG.15. The computer system 1500 may include one or more processors 1510 andone or more non-transitory computer-readable storage media (e.g., memory1520 and one or more non-volatile storage media 1530). The processor1510 may control writing data to and reading data from the memory 1520and the non-volatile storage device 1530 in any suitable manner, as theaspects of the invention described herein are not limited in thisrespect. To perform functionality and/or techniques described herein,the processor 1510 may execute one or more instructions stored in one ormore computer-readable storage media (e.g., the memory 1520, storagemedia, etc.), which may serve as non-transitory computer-readablestorage media storing instructions for execution by the processor 1510.

In connection with techniques described herein, code used to, forexample, generate instructions that, when executed, cause an additivefabrication device to operate one or more light sources (includingturning the light source on and off as discussed in relation to FIG.11), light directing components associated with such light sources(e.g., computer adjustable mirrors, such as mirror galvanometers),sensors, move an exposure source beneath a film surface, measure a forceapplied to a build platform, move a build platform, sense a position ofthe build platform, and/or adjust tension of a film system may be storedon one or more computer-readable storage media of computer system 1500.Processor 1510 may execute any such code to perform any of theabove-described techniques as described herein. Any other software,programs or instructions described herein may also be stored andexecuted by computer system 1500. It will be appreciated that computercode may be applied to any aspects of methods and techniques describedherein. For example, computer code may be applied to interact with anoperating system to transmit instructions to an additive fabricationdevice through conventional operating system processes.

The various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of numerous suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a virtual machine or a suitable framework.

In this respect, various inventive concepts may be embodied as at leastone non-transitory computer readable storage medium (e.g., a computermemory, one or more floppy discs, compact discs, optical discs, magnetictapes, flash memories, circuit configurations in Field Programmable GateArrays or other semiconductor devices, etc.) encoded with one or moreprograms that, when executed on one or more computers or otherprocessors, implement the various embodiments of the present invention.The non-transitory computer-readable medium or media may betransportable, such that the program or programs stored thereon may beloaded onto any computer resource to implement various aspects of thepresent invention as discussed above.

The terms “program,” “software,” and/or “application” are used herein ina generic sense to refer to any type of computer code or set ofcomputer-executable instructions that can be employed to program acomputer or other processor to implement various aspects of embodimentsas discussed above. Additionally, it should be appreciated thataccording to one aspect, one or more computer programs that whenexecuted perform methods of the present invention need not reside on asingle computer or processor, but may be distributed in a modularfashion among different computers or processors to implement variousaspects of the present invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically, the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in non-transitory computer-readablestorage media in any suitable form. Data structures may have fields thatare related through location in the data structure. Such relationshipsmay likewise be achieved by assigning storage for the fields withlocations in a non-transitory computer-readable medium that conveyrelationship between the fields. However, any suitable mechanism may beused to establish relationships among information in fields of a datastructure, including through the use of pointers, tags or othermechanisms that establish relationships among data elements.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

For instance, various modules within an additive fabrication device havebeen described with reference to a particular combination of moduleswith a particular additive fabrication technique (namely,stereolithography). It will be appreciated, however, that some of thesemodules may also be applied in other types of additive fabricationdevices. For example, the exposure module 109 shown in FIGS. 1A-1D orexposure module 209 shown in FIG. 2A may be deployed in a SelectiveLaser Sintering (SLS) device to melt or otherwise consolidate material.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

The above-described embodiments of the technology described herein canbe implemented in any of numerous ways. For example, the embodiments maybe implemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers. Such processorsmay be implemented as integrated circuits, with one or more processorsin an integrated circuit component, including commercially availableintegrated circuit components known in the art by names such as CPUchips, GPU chips, microprocessor, microcontroller, or co-processor.Alternatively, a processor may be implemented in custom circuitry, suchas an ASIC, or semi-custom circuitry resulting from configuring aprogrammable logic device. As yet a further alternative, a processor maybe a portion of a larger circuit or semiconductor device, whethercommercially available, semi-custom or custom. As a specific example,some commercially available microprocessors have multiple cores suchthat one or a subset of those cores may constitute a processor. Though,a processor may be implemented using circuitry in any suitable format.

The above-described techniques may be embodied as a computer readablestorage medium (or multiple computer readable media) (e.g., a computermemory, one or more floppy discs, compact discs (CD), optical discs,digital video disks (DVD), magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other tangible computer storage medium) encoded with one ormore programs that, when executed on one or more computers or otherprocessors, perform methods that implement the various embodiments ofthe invention discussed above. As is apparent from the foregoingexamples, a computer readable storage medium may retain information fora sufficient time to provide computer-executable instructions in anon-transitory form. Such a computer readable storage medium or mediacan be transportable, such that the program or programs stored thereoncan be loaded onto one or more different computers or other processorsto implement various aspects of the present invention as discussedabove. As used herein, the term “computer-readable storage medium”encompasses only a non-transitory computer-readable medium that can beconsidered to be a manufacture (i.e., article of manufacture) or amachine. Alternatively or additionally, the invention may be embodied asa computer readable medium other than a computer-readable storagemedium, such as a propagating signal.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method. The acts performed aspart of the method may be ordered in any suitable way. Accordingly,embodiments may be constructed in which acts are performed in an orderdifferent than illustrated, which may include performing some actssimultaneously, even though shown as sequential acts in illustrativeembodiments.

Further, some actions are described as taken by a “user.” It should beappreciated that a “user” need not be a single individual, and that insome embodiments, actions attributable to a “user” may be performed by ateam of individuals and/or an individual in combination withcomputer-assisted tools or other mechanisms.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value. The term“substantially equal” may be used to refer to values that are within 20%of one another in some embodiments, within 10% of one another in someembodiments, within 5% of one another in some embodiments, and yetwithin 2% of one another in some embodiments.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. An additive fabrication device configured to formlayers of solid material on a build platform, the additive fabricationdevice comprising: a container; a build platform; at least one forcesensor configured to measure a force applied to the build platform; andat least one processor configured to: form a layer of material incontact with the container, and in contact with the build platformand/or a previously formed layer of material; measure, using the atleast one force sensor, a length of time taken to separate the layer ofmaterial from the container; and control motion of the build platformbased at least in part on the measured length of time taken to separatethe layer of material from the container.
 2. The additive fabricationdevice of claim 1, wherein controlling the motion of the build platformbased at least in part on the measured length of time taken to separatethe layer of material from the container comprises positioning the buildplatform so that the layer of material is at a predetermined distancefrom the container.
 3. The additive fabrication device of claim 2,wherein controlling the motion of the build platform based at least inpart on the measured length of time taken to separate the layer ofmaterial from the container further comprises waiting for a time periodsubsequent to positioning the build platform so that the layer ofmaterial is at a predetermined distance from the container, wherein atleast one processor is further configured to determine the time periodbased at least in part on the measured length of time taken to separatethe layer of material from the container.
 4. The additive fabricationdevice of claim 1, wherein the at least one processor is furtherconfigured to select a sequence from amongst a plurality of sequences ofpredetermined motions of the build platform based at least in part onthe measured length of time taken to separate the layer of material fromthe container, and wherein controlling the motion of the build platformbased at least in part on the measured length of time taken to separatethe layer of material from the container comprises controlling themotion of the build platform in accordance with the selected sequence.5. The additive fabrication device of claim 1, further comprising awiper configured to move through the container, wherein the at least oneprocessor is further configured to control motion of the wiper based atleast in part on the measured length of time taken to separate the layerof material from the container.
 6. The additive fabrication device ofclaim 1, further comprising: an actuator arranged to move the buildplatform linearly in a first direction; and a deformable mountingstructure on which the actuator is mounted, the deformable mountingstructure arranged to allow the actuator to move in the first directionwhen the deformable mounting structure deforms, wherein the at least oneforce sensor is configured to measure the force applied to the buildplatform by detecting an amount of deformation of the deformablemounting structure.
 7. The additive fabrication device of claim 6,wherein the deformable mounting structure is mounted to at least onerigid structure.
 8. The additive fabrication device of claim 7, whereinthe deformable mounting structure is mounted to at least a first rigidstructure and a second rigid structure such that the actuator is mountedto the deformable mounting structure between the first rigid structureand the second rigid structure.
 9. The additive fabrication device ofclaim 6, wherein the at least one sensor includes at least one inductivesensor.
 10. The additive fabrication device of claim 9, furthercomprising an inductive reference target attached to the deformablemounting structure.
 11. The additive fabrication device of claim 6,wherein the deformable mounting structure comprises a metal sheet. 12.The additive fabrication device of claim 6, further comprising athreaded rod to which the build platform is coupled, and wherein theactuator is arranged to rotate the threaded rod about its axis.
 13. Theadditive fabrication device of claim 1, wherein measuring, using the atleast one force sensor, the length of time taken to separate the layerof material from the container comprises identifying, using the at leastone processor, a moment at which the layer of material separates fromthe container by identifying a spike in a plurality of forcemeasurements taken over time by the at least one force sensor.
 14. Theadditive fabrication device of claim 1, wherein measuring, using the atleast one force sensor, the length of time taken to separate the layerof material from the container comprises identifying, using the at leastone processor, a moment at which the layer of material separates fromthe container by detecting when force measurements taken over time bythe at least one force sensor are within a predetermined range of forcemeasurements.
 15. A method of operating an additive fabrication deviceconfigured to form layers of solid material on a build platform, eachlayer of material being formed in contact with a container in additionto the build platform and/or a previously formed layer of material, themethod comprising: forming a layer of material in contact with thecontainer, and in contact with the build platform and/or a previouslyformed layer of material; measuring, using at least one force sensor, alength of time taken to separate the layer of material from thecontainer; and controlling motion of the build platform based at leastin part on the measured length of time taken to separate the layer ofmaterial from the container.
 16. The method of claim 15, whereincontrolling the motion of the build platform based at least in part onthe measured length of time taken to separate the layer of material fromthe container comprises positioning the build platform so that the layerof material is at a predetermined distance from the container.
 17. Themethod of claim 16, wherein controlling the motion of the build platformbased at least in part on the measured length of time taken to separatethe layer of material from the container further comprises waiting for atime period subsequent to positioning the build platform so that thelayer of material is at a predetermined distance from the container, andwherein the method further comprises determining, using at least oneprocessor, the time period based at least in part on the measured lengthof time taken to separate the layer of material from the container. 18.The method of claim 15, further comprising, selecting, using at leastone processor, a sequence from amongst a plurality of sequences ofpredetermined motions of the build platform based at least in part onthe measured length of time taken to separate the layer of material fromthe container, and controlling the motion of the build platform inaccordance with the selected sequence.
 19. The method of claim 15,wherein measuring, using the at least one force sensor, the length oftime taken to separate the layer of material from the containercomprises identifying, using at least one processor, a moment at whichthe layer of material separates from the container by identifying aspike in a plurality of force measurements taken over time by the atleast one force sensor.
 20. The method of claim 15, wherein measuring,using the at least one force sensor, the length of time taken toseparate the layer of material from the container comprises identifying,using at least one processor, a moment at which the layer of materialseparates from the container by detecting when force measurements takenover time by the at least one force sensor are within a predeterminedrange of force measurements.